The power station of fluctuating electrical capacity may particularly be a photovoltaic power station, but also, by way of example, a wind, tidal or a cogeneration unit. The “fluctuating” statement with regard to the capacity of the power station means, in this context, that its capacity, i.e. its present ability to output electric power, is determined by external parameters such as insolation, wind activity, tidal range, heat requirement, etc. and fluctuates as a result, specifically irrespective of the present power requirement in the limited AC grid to which the power station is connected.
The limited AC grid is particularly what is known as an island system, which does not have a connection to a superordinate stabilizing AC grid. The system former connected to the AC grid may, however, fundamentally also have a connection of limited power to the superordinate AC grid. The limited AC grid accordingly has no more than one connection of limited power to a superordinate AC grid. Frequently, the system former will be a permanently activated combustion generator that has a constant capacity at the level of its rated or maximum power. Formation of the system former is alternatively also possible by a hydroelectric generator or a battery inverter.
Depending on the power requirement of the loads connected to the AC grid, frequently at least one further power source, particularly a combustion generator, is activated, in addition to the system former, by a power source controller as required. Typically, there are multiple further power sources that are activated by the power source controller in increasing number as required. In this case, the power source controller keeps a reserve power to hand so that the currently activated power sources with an increased power output can react to brief rises in the power consumption of the loads or to an additional power consumption by activating additional loads. Activation of additional combustion generators does not allow these kinds of rapid rises in power consumption.
With any fluctuation in the power consumption for loads, particularly all voltage-setting system formers are given the task of keeping the mains voltage of the AC grid stable, i.e. as constant as possible in terms of amplitude and frequency.
The power station of fluctuating electrical capacity reduces the fuel requirement of all combustion generators connected to the limited AC grid as power sources and any other energy requirement of other power sources connected to the limited AC grid that is not covered by regenerative energy sources by covering part of the power consumption of the connected loads. In this case, it should be borne in mind that the power station of fluctuating electrical capacity does not lead to destabilization of the AC grid by virtue of its supplying power to the AC grid. As such, an uncontrolled supply of power by the power station can lead to power being supplied to activated combustion generators, for example, which normally results in emergency shutdown thereof. This can cause the whole AC grid to collapse.
Combustion generators connected as power sources to the AC grid are typically diesel generators, even if other combinations of an internal combustion engine and a generator, which in this case are referred to as combustion generators for short, are possible. Although diesel generators, when put to appropriate use, have good efficiency and good cost effectiveness, they need a few seconds before they are activated to the extent that they can supply power to the AC grid. The electric power supplied by photovoltaic power stations, for example, can be varied more quickly in principle, even more quickly than in the case of an already activated combustion generator, which is afflicted with considerable mechanical moments of inertia.
A method for operating a photovoltaic power station in order to reduce a power consumption of a diesel generator by supplying power from the photovoltaic power station is described on the website www.donauer.eu of Donauer Solartechnik Vertriebs GmbH. In this case, what is known as DONAUER diesel hybrid energy management is intended to ensure a stable, economical and sustainable supply of energy without the need for batteries as buffer stores for electric power, by virtue of a diesel generator always being operated at an optimum operating point with efficient fuel combustion. More information concerning how this is intended to be achieved is not provided, however.
DE 10 2011 103 600 A1 discloses a method for controlling an installation or machine for the purpose of optimum utilization of an energy source. In this case, the installation or machine can be supplied with power from a photovoltaic power station. The time profile for the power of the photovoltaic power station is forecast, with a cloud cover forecast being produced with the aid of satellites. The installation or machine is then controlled such that it performs a process that is begun at a starting time at which at least a greatest possible portion of the power needed for the process is produced by the photovoltaic power station.
DE 10 2010 046 744 A1 discloses a method for a photovoltaic power station with load management in island mode. A first energy source consisting of an arrangement of photovoltaic modules and of a second energy source supplies to a DC input of an island inverter, the output of which supplies power to an AC load. In this case, the input of electric power is provided solely using the first energy source, as long as the maximum power point on the current/voltage characteristic of the photovoltaic modules has not yet been reached. When the maximum power point and a further consumption load increase in the AC load are reached, the photovoltaic modules continue to be operated at the maximum power point, and the energy input that is missing for the full supply of energy to the AC load is provided by the second energy source. This second energy source may be formed by a public power supply system or by a combustion generator.
DE 10 2011 106 877 A1 discloses an arrangement with a photovoltaic generator that is connectable to the input terminals of an inverter. The output of the inverter is connected to a supply system. The arrangement moreover comprises an asynchronous machine that is connected in parallel with the output of the inverter and that is driven by an internal combustion engine. The aim of the arrangement is to allow the use of asynchronous machines, which are cheap in comparison with synchronous machines and whose unfavorable power factor cos phi is compensated for by reactive power control using electronically regulated IGBTs of the inverter. A combustion generator with an asynchronous machine directly linked to an AC grid is not suitable as a system former. In order to span the period between a time at which power is demanded from the synchronous machine by the operator of the supply system and the time from which the asynchronous machine actually provides the demanded power after the internal combustion engine is started, a battery is therefore provided that is connectable to the input of the inverter.
EP 1 323 222 B1 discloses a method for operating an island system having at least one first energy producer in the form of a wind power station, a synchronous machine as a system former and an electrical element as an intermediate storage. The wind power station is regulated such that it always produces only the electric power needed, which is made up of the consumption of the electric power in the system and the power requirement for charging the electrical element as an intermediate storage. An internal combustion engine that can drive the synchronous machine via a coupling is activated only when the power that is output by the wind power station and/or by the electrical intermediate storage drops below a prescribable threshold value for a prescribable period of time. When the internal combustion engine is not activated, the synchronous machine operates in motor mode, drawing its drive energy from the wind power station. The synchronous machine is not only a system former but is also a reactive power and short-circuit current generator, flicker filter and voltage regulator of the island system.
US 2008/0278003 A1 discloses a system for the uninterruptable supply of power to a load that has a regenerative energy source in the form of a photovoltaic power station and a battery as a storage for electric power. The system maintains the supply of power to the load when an AC grid fails, particularly until a combustion generator is activated as a substitute power supply.
C. A. Hernandez-Aramburo et al.: Fuel Consumption Minimization of a Microgrid” IEEE Transactions on Industry Applications, vol. 41, no. 3, pages 673-681, May/June 2005, discloses a method for operating photovoltaic generators and wind generators that, besides a system former, at least one combustion generator, which is activated or deactivated by a generator controller as required, and at least one load, are connected to a limited AC grid. A cost optimization model, with the secondary condition that a reserve power currently available in the AC grid observes a minimum value, sets the power available from photovoltaic generators and wind generators at 80% and 40%, respectively, of their rated power. To provide a rising power requirement, the photovoltaic generators and wind generators, which require no fuel, are initially run up to their present maximum powers. In order to provide reserve power and in order to cover power requirement that can no longer be covered without fuel, combustion generators are then connected in a combination that is cost-optimized for the respective power requirement. The powers of the photovoltaic generators and wind generators remain at their present maximum powers in this case.
EP 2 503 146 A1 discloses a method and an apparatus for controlling the operation of an energy production installation during isolation from a superordinate AC grid. In concrete terms, this involves an energy production installation that comprises at least one wind power station, and the mode of operation of the energy production installation in the event of failure of the superordinate AC grid, to which energy is supplied otherwise. For this mode of operation, an external generator is connected by means of a cable to auxiliary equipment of the wind power station in order to provide this auxiliary equipment, such as the setting devices for the rotor blades of the wind power station, for example, with active power. A transformer connects the cable not only to the auxiliary setup but also to an inverter of the respective wind power station. The inverter is used to provide reactive power in order to operate the external generator in a stable range of the PQ level, i.e. within the area defined by the possible active power of the generator plotted over the possible reactive power of the generator. The background is that a standard diesel generator can become unstable for particular ratios of active power to reactive power above 20% reactive power for its nominal power and that an acceptable operating range on the PQ level lies approximately between the straight lines representing a power factor of 1.0 and a power factor of 0.8. The reactive power produced using the inverter of the wind power station is used to keep the operating point of the external generator in this range.